This invention relates to a glass-ceramic substrate for an information storage medium and, more particularly, to a glass-ceramic substrate for an information storage medium such as a magnetic disk made of a glass-ceramic having improved super flatness of a surface of the substrate suitable for use in the near contact recording system or the contact recording system, a high Young's modulus and a low specific gravity capable of coping properly with a high-speed rotation, excellent mechanical properties, and a range of coefficient of thermal expansion matching with coefficients of thermal expansion of constituent elements of a drive device for the information storage medium. The invention relates also to a method for manufacturing the same and also to an information storage medium using this glass-ceramic substrate. In this specification, the term "information storage medium" means an information storage medium in the form of a disk and includes fixed type hard disks, removable type hard disks and card type hard disks used respectively for so-called "hard disks" for personal computers and storage of information in a network and other information storage medium in the form of a disk which can be used for storage of data in, e.g., digital video cameras and digital cameras.
Recent development of personal computers for multi-media purposes and digital video cameras and digital cameras which requires handling of a large amount of data has increased a demand for an information storage device of a higher recording capacity. As a result, for increasing the recording density, it is necessary in an information storage medium to increase its bit and track density and reduce the size of a bit cell. In conformity with the reduction in the size of the bit cell, a magnetic head performs its operation in closer proximity to the surface of a disk. As the magnetic head performs its operation in a near-contact state or contact state against the disk surface, it becomes important that a substrate has a super flat surface.
For satisfying this requirement, aside from the conventional landing zone system, development of a ramp loading system is under way according to which a magnetic head is completely in contact with the surface of a medium except for starting and stopping of the magnetic head when the magnetic head is moved away from the surface of the medium. Accordingly, a current requirement for a substrate for a magnetic information storage medium is a smoother surface.
For coping with the increase in the amount of information to be stored, a technical development is under way for a higher speed transfer of information by a higher speed rotation of a magnetic information storage medium used for a magnetic information storage device. As the number of revolution of a medium increases, deflection and deformation of the medium occur and this gives rises to a requirement for a higher Young's modulus and a lower specific gravity. Further, in addition to the conventional fixed type hard disks, information storage media such as a removable type hard disks and card type hard disks have been proposed and put into practice and application of digital video cameras and digital cameras for various uses have been started. Further uses of the medium for mobiles are expanding and, accordingly, mechanical strength of the substrate has increased its importance.
Development for new uses of information storage media for mobiles (e.g., APS cameras, portable telephones, digital cameras, digital video cameras and card drive), desk top PC (hard disk drive), servers (hard disk drive) and new high recording density media (e.g., perpendicular magnetic recording storage medium, Island magnetic storage medium and semiconductor memory) have also been started and, therefore, higher properties will be required for substrates for these information storage media.
Known in the art of magnetic disk substrate materials is aluminum alloy. The aluminum alloy substrate, however, has projections or spot-like projections and depressions on the substrate surface during polishing due to various defects of the material and, therefore, is not sufficient as a substrate for a high recording density storage medium in flatness and smoothness. Besides, since aluminum alloy is a soft material and has a low Young's modulus and surface hardness, vibration of the substrate takes place during a high-speed rotation of the medium with resulting deformation of the medium. Difficulty also arises in making the information storage medium thinner. Further, damage of the medium by contact with a head is liable to occur. Thus, the aluminum alloy substrate cannot sufficiently cope with the requirements for a high-speed recording.
As materials for overcoming the above problems of the aluminum alloy substrate, known in the art are chemically tempered glasses such as alumino-silicate glass (SiO.sub.2 --Al.sub.2 O.sub.3 --Na.sub.2 O). These materials, however, have the following disadvantages: (1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner. (2) Since the glass contains Na.sub.2 O as an essential ingredient, the glass has the problem that the film forming characteristics of the medium is deteriorated and, for preventing diffusion of Na.sub.2 O, it becomes necessary to apply a barrier coating over the entire surface of the substrate. This prevents stable production of the product at a competitive cost.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, the glass-ceramics of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system disclosed in U.S. Pat. No. 5,626,935 containing lithium disilicate (Li.sub.2 O.multidot.2SiO.sub.2) and .alpha.-quartz (.alpha.-SiO.sub.2) as main crystal phases is an excellent material as a material textured over the entire surface in which, by controlling the grain diameter of globular crystal grains of .alpha.-quartz, the conventional mechanical texturing or chemical texturing can be omitted and the surface roughness after polishing (Ra) can be controlled within a range from 15 .ANG. to 50 .ANG.. A surface roughness Ra (arithmetic mean roughness) which is sought today, however, is 5.0 .ANG. or below, preferably 3.0 .ANG. or below, and more preferably 2.0 .ANG. or below and this prior art glass-ceramic cannot sufficiently cope with the requirement for the low glide height necessitated by the rapidly increasing recording density which requires such extremely small surface roughness (Ra), Further, no discussion or suggestion has been made in the patent about crystal grain diameter degree of crystallization and mechanical strength including Young's modulus and specific gravity which will be described later in this specification.
Japanese Patent Application Laid-open Publication No. Hei 10-45426 discloses a SiO.sub.2 --Li.sub.2 O--K.sub.2 O--MgO--ZnO--P.sub.2 O.sub.5 --Al.sub.2 O.sub.3 system glass-ceramic or a SiO.sub.2 --Li.sub.2 O--K.sub.2 O--MgO--ZnO--P.sub.2 O.sub.5 --Al.sub.2 O.sub.3 --ZrO.sub.2 system glass-ceramic suitable for laser texturing which contains, as its predominant crystal phase or phases, at least one of lithium disilicate (Li.sub.2 O.multidot.2SiO.sub.2), a mixed crystal of lithium disilicate and .alpha.-quartz (.alpha.-SiO.sub.2), and a mixed crystal of lithium disilicate and .alpha.-cristobalite (.alpha.-SiO.sub.2). Surface roughness Ra which is sought today, however, is 5.0 .ANG. or below, preferably 3.0 .ANG. or below, more preferably 2.0 .ANG. or below as described above and the prior art glass-ceramic substrates cannot sufficiently cope with the requirement for a reduced glide height which is necessitated by the rapidly increasing recording density. Besides, no discussion has been made in the prior art publication as to crystal grain diameter, degree of crystallization, and mechanical strength including Young's modulus and specific gravity.
Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a magnetic disk substrate made of a glass-ceramic of a SiO.sub.2 --Al.sub.2 O.sub.3 --Li.sub.2 O system having predominant crystal phases of lithium disilicate (Li.sub.2 O.multidot.2SiO.sub.2) and .beta.-spodumene (Li.sub.2 O.multidot.Al.sub.2 O.sub.3.multidot.4SiO.sub.2). This glass-ceramic, however, contains, as its predominant crystal phase, .beta.-spodumene which has a negative thermal expansion characteristic which causes the substrate to have a negative thermal expansion characteristic and, in this glass-ceramic, growth of SiO.sub.2 crystals such as .alpha.-quartz (.alpha.-SiO.sub.2) and .alpha.-cristobalite (.alpha.-SiO.sub.2) which have a positive thermal expansion characteristic and thereby cause the substrate to have a positive thermal expansion characteristic is extremely restricted. The arithmetic mean surface roughness of this glass-ceramic after polishing is defined as 20 .ANG. or below but the arithmetic mean surface roughness disclosed in examples is a rough one of 12 .ANG.-17 .ANG. which fails to reach the above described desired surface roughness and, therefore, this glass-ceramic cannot cope sufficiently with the requirement for the low glide height of a magnetic head. Besides, the material which grows a crystal having a negative thermal expansion characteristic as a predominant crystal phase apparently adversely affects the substrate in difference in the coefficient of thermal expansion with respect to component parts of a drive device. Further, since this glass-ceramic requires a high temperature of 820.degree. C. to 920.degree. C. for crystallization which prevents a large scale production of the product at a competitive cost. Furthermore, no discussion has been made in the publication as to mechanical strength including crystal grain diameter, degree of crystallization and mechanical strength.
International Publication WO97/01164 which includes the above described Japanese Patent Application Laid-open Publication No. Hei 9-35234 discloses a glass-ceramic for a magnetic disk in which temperature for crystallization is reduced (680.degree. C.-770.degree. C.). A sufficient improvement however cannot be achieved in this substrate. Besides, crystals grown in all examples disclosed are .beta.-eucryptite (Li.sub.2 O.multidot.Al.sub.2 O.sub.3.multidot.2SiO.sub.2) which has a negative thermal expansion characteristic and this adversely affects difference in the coefficient of thermal expansion with respect to component parts of a drive device. Further, no discussion has been made in this publication as to crystal grain diameter, degree of crystallization and mechanical strength.
It is, therefore, an object of the invention to eliminate the above described disadvantages of the prior art and provide a glass-ceramic substrate for an information storage medium having a super flat surface characteristic capable of coping with recording at a low glide height or contact recording of a magnetic head necessitated by increase in the recording capacity of an information storage medium, having properties capable of coping with a high-speed rotation of the medium necessitated by increase in the information transfer speed, and having a high mechanical strength adapted for uses as an information storage medium for mobiles.
It is another object of the invention to provide a method for manufacturing the glass-ceramic substrate.
It is another object of the invention to provide an information storage medium employing this glass-ceramic substrate.
More specifically, requirements for the glass-ceramic substrate according to the invention are summarized as follows:
(1) As the recording density increases, it becomes necessary to reduce the size of the bit cell. As the bit cell is reduced, the magnetic head performs its operation in closer proximity to the surface of an information storage medium. In case the magnetic head performs its operation at such a low glide height (near contact state) or in contact with the surface of the medium, the substrate is required to have a super flat surface having a surface roughness Ra of 5.0 .ANG. or below, preferably 3.0 .ANG. or below, and more preferably 2.0 .ANG. or below.
(2) As the substrate is required to have a super flat surface, crystal grains in the substrate are required, for standing a precision polishing which is conducted almost in the level of atoms, to be of a specific crystal type which has chemical durability and physical properties and to have crystal grain diameter, crystal shape and degree of crystallization which can sufficiently cope with the high precision polishing.
(3) As the recording density increases, the number of rotation of the medium must be increased. For coping with such high-speed rotation of the medium, a higher Young's modulus and a lower specific gravity of the substrate become important.
(4) As uses of an information storage medium are expanded and particularly uses for mobiles are developed, mechanical strength of the substrate becomes important.
(5) Since positioning between a magnetic head and an information storage medium requires a high precision, the substrate and component parts of a drive device for the medium must have a size of a high precision. Accordingly, difference in a coefficient of thermal expansion between the substrate and the component parts of the drive device should be minimized in accordance with environmental temperature at which the substrate is used.